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Increasing malaria during an eradication programme
555 TRANSACTIONS OF THE ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE.
Vol. 64.
No. 4.
1970.
INCREASING MALARIA DURING AN ERADICATION PROGRAMME A study on the tea estates in A s s a m , 1964-68
A. B. GILROY Ross Institute of Tropical Hygiene, India Branch In 1966 and 1967 outbreaks of malaria were reported from tea estates in the Brahmaputra Valley of Assam. By 1968 the loci had increased in number, in the degree of endemicity and in geographical distribution, affecting estates scattered along 250 miles of the valley. In 1968, 269 estates were members of the Assam Branch of the Indian Tea Association. The total area of tea was 99,500 hectares and the resident population, workers and dependants, numbered 790,000. The Assam Branch is divided into three administrative zones (see map). q. ~
I BHUTAN
)
I
/L". !fJ
....
[I!],"l li/i
ll.
,-
~/.--
31,
I
CHINA
FRONTIER
"~-
a\\~,"
/
'~O~ONAR,I
" I
, ,
x~ : ,
(
TEZPUR =q~l., . '
BURMA
,. NAGALAND
N/) x
Lj Fm.
x
.-.-
,----.,
t
io~
mille~
Map of Assam showing tea districts and localities with highest parasite incidences
Zone I, the most easterly, is on the south bank of the Brahmaputra and extends towards the foothills of the Tirap Frontier Division of the North East Frontier Agency. Ir has 44yo of the tea area and 47% of the estate population. Zone 2, also on the south bank, extends to the border of Nagaland; it has 27% of the area of tea and 29% of the population. Zone 3 is on the north of the Brahmaputra and extends to the borders
556
INCREASING MALARIA DURING AN ERADICATION PROGRAMME
of the North East Frontier Agency and Bhutan. It has 29% of the tea area and 24% of the population. Among the rice fields, forests, rivers and swamps that make up the plains districts of Assam, tea estates are only a small part. History o f m a l a r i a control on tea estates
Malaria control started in the early 1930s. At the outbreak of World War II, larval control by flushing (MacDONALD, 1939) and species sanitation (RaMsAY and MaC.DONALD, 1936) had considerably reduced the amount of malaria. Wartime shortages of men and materials put a stop to larval control and when the war ended malaria had returned to its former high level. However, first D D T then BHC became available and there was no thought of returning to the relatively ineffective methods of larval control when a direct attack on the adult mosquito became possible. In 1950 a uniform regimen of insecticide spraying was recommended. BHC in a concentration of 11 mg. of gamma BHC per 1 square foot was sprayed in all human habitations and adjacent cattle sheds every 5 weeks during the transmission season, particular importance being given to an early first round to be completed by the end of March when breeding of the vector, Anopheles minimus, was to be expected. BHC was recommended in preference to D D T because its lethal effects were less adversely affected by adsorption on mud walls and because the short intervals between rounds would offset the covering of the insecticide deposit when wails were re-plastered with mud and cowdung, as is customary throughout rural India. The success of the estates' programme is indicated by the Annual Parasite Incidence. In 1954, when the programme of BHC spraying was widely adopted on estates the incidence was 460 per 10,000. In 1963, the last year before estates handed over control to the National Malaria Eradication Programme, it was 1 per 10,000. The National Malaria Eradication Programme (NMEP) was introduced in Assam in 1958, D D T being used in a concentration of 100 rag. per 1 square foot applied twice a year. Tea estates were not included in the spraying programme but could elect to be supplied with D D T , intended to replace BHC which could be held in reserve lest the vector develop resistance to D D T . During 1964 the N M E P took over full responsibility for the attack phase on tea estates. Malaria control that had been successfully carried out for many years by estates with their own staff, equipment, and insecticide was supplanted by malaria eradication which, in 1968, was in its fifth year on tea estates. Some districts were placed in the consolidation phase in 1965 and 1966 but in 1968 reverted, at least in part, to the attack phase. Qnestionaries revealed no consistent pattern and in 1968 an estate might be sprayed once or even twice while neighbouring estates were not sprayed at all. Malaria t r a n s m i s s i o n in A s s a m
The principal vector is A. minimus and transmission has been reported throughout the year except for the winter months of December to March. Breeding is prolific after the early light rains of Spring but decreases when heavy monsoon rains flood the streams and channels. Cases of malaria reach a peak in September or October and thereafter rapidly decline. In three "indicator" estates that are included in this study, in a population of 10,000 there were 16 cases of malaria between January and March 1968, 524 cases in August and September and a total of 836 in the year. A. leucosphyrus (probably A. balabacensis) is a proved secondary vector on a few estates that are close to heavily forested foothills. The importance of A. philippinensis as a vector is still uncertain.
A. B. GILROY
557
Investigations and results To measure the increase of malaria, and its incidence in 1968 on estates that had been free from malaria for several years, a questonary was sent to members of the Assam Branch of the Indian Tea Association asking for this information: The total resident population, workers and dependants The number of fever cases in 1968 The number of blood smears made and examined in 1968 The number of smears reported as positive by the NMEP in each year, 1964-1968 The starting and finishing dates for D D T spraying in 1968 Details of focal spraying of estates in the consolidation phase Details and results of mass blood surveys. Completed questionaries were received from 239 of the 269 tea estates. In Zone 1, the replies covered 94% of the area of tea, in Zone 2, 100% and in Zone 3, 77%. For the total area the coverage was 91%. Briefly, 31 estates reported at least one case of malaria in 1968 but none in 1967 while a further 52 reported an increase of cases in 1968. As no estate has a population of 10,000, and most have" fewer than 5,000, even one case implies an Annual Parasite Incidence (API) exceeding 1 per 10,000. The data are first tabulated for the three Zones of the Assam Branch (Table I). This gives a picture unobscured by detail but it is not evident that only a few estates in each Zone were seriously affected by malaria in 1968. To show this the data are presented for the most malarious estates grouped according to their localities (Table II). TABLE I.
Zone
Population
1
335,734
204,836
174,837
Total
715,407
Malaria cases and annual parasite incidence by Zones Cases & API*
1968
1967
1966
1965
1964
Cases
738
731
567
268
181
API
22.0
21 '8
16 '9
8"0
5-4
Cases
1430
865
711
573
518
API
69"3
42"2
34"7
28"0
25-3
Cases
85
113
36
34
174
API
4"9
6.5
2.1
1"9
10"0
Cases
2253
1709
1314
875
873
API
31-5
23"9
18 "4
12"2
I
12.2
*Annual parasite incidence, cases per 10,000
Malaria cases and the annual parasite incidence in the Zones Table I shows the numbers of cases and the API in each of the three Zones from 1964 to 1968. Combining the Zones, 873 cases in 1964 had increased to 2,253 in 1968; of the total cases in 1968, 33% were in Zone 1, 63% in Zone 2 and only 4% in Zone 3. The API for the combined Zones had increased from 12.2 per 10,000 in 1964 to 31.5 in 1968. The increase was most marked in Zone 2. Why Zone 3 should be relatively D
558
INCREASING MALARIA DURING AN ERADICATION PROGRAMME
malaria-free is not evident from the questionaries. As recently as 1957 the malaria morbidity rate in one district was 18 per 1,000 (GILROY and SCOTT, 1958). The same district reported a morbidity of 0.4 per 1,000 in 1968. The yearly increases of the API are emphasized if the 1964 incidence is taken as unity and the 1968 incidence is expressed as a proportion. In Zone 1, the API in 1968 was 4.1 times greater than in 1964, in Zone 2 it was 2.8 times greater and only in Zone 3 was there a fall from 1.0 in 1964 to 0.5 in 1968. The Tenth Report of the W H O Expert Committee on Malaria (WHO, 1964) reviewing criteria for the start of consolidation recommends that the permissible API should not exceed 0.1 per 1,000 (1 per 10,000). In the Indian programme an API of 2 per 10,000 is accepted as an indication that if it is exceeded in areas in the consolidation phase, they should revert to the attack phase provided that the loci of indigenous transmission are scattered and cannot be cleared by routine remedial measures (DmR, 1968). In 1968 more than 100 tea estates recorded an API higher than 2 per 10,000; on many it was twenty and thirty times higher. Table I shows that in Zone 1, 181 cases in 1964 had increased to 738 in 1968. The increase in Zone 2 was more striking, 518 cases in 1964 and 1,430 in 1968. A total of 2,253 cases for the three Zones in a population of 790,000 would have been unbelievably few 40 years ago but not in the fifth year of the National Eradication Programme. M a l a r i a on s e l e c t e d e s t a t e s o f h i g h e n d e m i c i t y
Table I I gives the number of cases and the annual parasite incidences for 38 estates selected because of their high API and grouped in 6 localities. In 1968 the lowest API was 47 per 10,000 and the highest 301. Of the total resident population of the three combined Zones, 16% live on these 38 estates which produced 87% of the malaria in 1968. For all 38 estates the API increased slowly from 46 per 10,000 in 1964 to 99 in 1967 and thence sharply to 174 in 1968. I f the API of 1964 is taken as unity, the annual increase was 1.2 times in 1965, 1.5 times in 1966, 2.2 times in 1967 and 3.8 times in 1968. Although malaria in 1968 was a serious problem on only 38 of the 239 estates that completed the questionary, two ominous facts appear, the API is increasing more sharply and these 6 localities are widely separated along 250 miles of the south bank of the Brahmaputra river. The questionaries do not give information to explain the considerable differences in the incidence of malaria in the groups, ranging in 1968 from 47 to 301 per 10,000. For that, epidemiological and entomological investigations would be necesasry. S u r v e i l l a n c e o n tea estates
Tea estates must by law provide hospitals and medical staff for workers and their families. Full use is made of medical services, people coming for treatment of the most minor ailment. This facilitates efficient passive case detection and treatment, both presumptive and radical. Microscope slides and anti-malarial drugs are supplied by the N M E P and blood smears made on the estates are collected by surveillance workers and examined in the N M E P laboratories. 61,512 blood smears from tea estates were examined in 1968 and 2,253 were positive for malaria, a slide positivity rate of 3.7%. The completeness of blood smear examination is indicated by the Annual Blood Examination Rate (ABER) which expresses the
A. B. GILROY
559
number of blood smears in the year as a percentage of the relevant population. DmR (1968) accepts an ABER between 5% and 10% of the population in hyperendemic areas as reasonable if remedial measures are prompt and adequate. TABLE II.
Malaria cases and annual parasite incidences for groups of selected estates
Locality of of group Population Doom Dooma
18,671
Sonari
20,526
Jorhat
30,223
Golaghat
11,859
Nowgong
24,879
Tezpur
Total
6,754
112,912
*API--Annual parasite incidence: cases per 10,000. Note. For convenience of description the localities of the groups are given the name of the nearest township. Table I I I gives data for passive surveiUance both for the six groups of 38 malarious estates and for the three Zones. The ABER was 6.9% in Zone 1, 10.8% in Zone 2 and 9.3% in Zone 3. The ABER will be reduced by the inclusion of estates where malaria is only a minor problem and where, consequently, little interest is taken in passive case detection. On the other hand, in the groups of seriously affected estates the ABER ranges between 13% and 28% compared with 10% or less in the Zones. The slide positivity rates are evidence of the wide extent and volume of the parasite reservoir. As shown in Table I I I in one group where 3,714 smears were examined, 14-9% were positive and of 22,054 smears made in Zone 2, 6.5% were positive.
INCREASINGMALARIADURINGAN ERADICATIONPROGRAMME
560
TABLE III. Passive case detection 1968 for the three Zones and selected estates of Table II
Population
Fever cases
No. of blood smears
Doom Dooma
18,671
3,714
3,714
20
14.9
Sonari
20,526
3,135
2,603
13
6.8
Jorhat
30,223
6,301
6,211
21
2-7
Golaghat
11,859
4,464
3,370
28
10"6
Nowgong
24,879
5,391
4,950
20
13-6
6,754
1,157
1,157
17
2.8
Zone 1
336,000
31,041
23,119
6"9
3.2
Zone 2
204,900
28,304
22,054
10.8
6"5
Zone 3
174,900
24,853
16,339
9-3
0-5
Locality of group
Tezpur
ABER* per cent
Slide positivity rate per cent
Zone
Note.
The groups are included in their Zones. *Annual blood examination rate---number of blood smears expressed as a percentage of the relevant population.
DDT spraying cycles in 1968 The first round of spraying was scheduled to start on 15 May and to be completed by the end of June; the second round was to be applied between the middle of August and the end of September. In the event the starting dates were seriously delayed and D D T was not applied until the transmission season was established and secondary cases were occurring. On many estates the first round was started after the second should have been finished and of 149 estates that were sprayed in 1968, 34 were sprayed for the first time in November when malaria transmission had ceased. Even had the proposed time-table been observed, a first round of D D T starting in the middle of May is too late to prevent transmission in April and the secondary cases that are the true origin of epidemics during eradication (MACDONALD 1957).
Summary Data obtained by questionary have established the increase, year by year, of malaria on tea estates. It has been measured by the almual parasite incidence and the extent of the parasite reservoir has been estimated by the annual slide positivity rate. The most seriously affected estates are confined to six localities, far apart and separated by villages and by tea estates where there has not been an increase of malaria. D D T spraying in 1968 was delayed until long after the transmission of malaria was established. Passive case detection was adequate and approached, and even exceeded, the standard of 10% of the population as measured by the annual blood examinauon rate. The increase of malaria over an ever wider area is affecting people whose immunity has been lost during years of freedom from attacks.
A. B. GILROY
561
REFERENCES DHIR, S. L. (1968). Bull. lndian Soc. Mal. Com. Dis., 5, 16. GILROY, A. B. & SCOTT,A. (1958). lndian ft. Malariol., 12, 165. MACOONALD, G. (1939). ft. Malar. lnst. India, 2, 63. (1957). The Epidemiology and Control of Malaria. London: Oxford University Press. RAMSAY, G. C. & MACDONALD,G. (1936). lndian reed. Gaz., 71, 699. WHO (1964). WHO Expert Committee on Malaria. Tech. Rep. Ser. No. 272. Geneva: World Health Organization.
Vol. 64.
No. 4.
1970.
INCREASING MALARIA DURING AN ERADICATION PROGRAMME A study on the tea estates in A s s a m , 1964-68
A. B. GILROY Ross Institute of Tropical Hygiene, India Branch In 1966 and 1967 outbreaks of malaria were reported from tea estates in the Brahmaputra Valley of Assam. By 1968 the loci had increased in number, in the degree of endemicity and in geographical distribution, affecting estates scattered along 250 miles of the valley. In 1968, 269 estates were members of the Assam Branch of the Indian Tea Association. The total area of tea was 99,500 hectares and the resident population, workers and dependants, numbered 790,000. The Assam Branch is divided into three administrative zones (see map). q. ~
I BHUTAN
)
I
/L". !fJ
....
[I!],"l li/i
ll.
,-
~/.--
31,
I
CHINA
FRONTIER
"~-
a\\~,"
/
'~O~ONAR,I
" I
, ,
x~ : ,
(
TEZPUR =q~l., . '
BURMA
,. NAGALAND
N/) x
Lj Fm.
x
.-.-
,----.,
t
io~
mille~
Map of Assam showing tea districts and localities with highest parasite incidences
Zone I, the most easterly, is on the south bank of the Brahmaputra and extends towards the foothills of the Tirap Frontier Division of the North East Frontier Agency. Ir has 44yo of the tea area and 47% of the estate population. Zone 2, also on the south bank, extends to the border of Nagaland; it has 27% of the area of tea and 29% of the population. Zone 3 is on the north of the Brahmaputra and extends to the borders
556
INCREASING MALARIA DURING AN ERADICATION PROGRAMME
of the North East Frontier Agency and Bhutan. It has 29% of the tea area and 24% of the population. Among the rice fields, forests, rivers and swamps that make up the plains districts of Assam, tea estates are only a small part. History o f m a l a r i a control on tea estates
Malaria control started in the early 1930s. At the outbreak of World War II, larval control by flushing (MacDONALD, 1939) and species sanitation (RaMsAY and MaC.DONALD, 1936) had considerably reduced the amount of malaria. Wartime shortages of men and materials put a stop to larval control and when the war ended malaria had returned to its former high level. However, first D D T then BHC became available and there was no thought of returning to the relatively ineffective methods of larval control when a direct attack on the adult mosquito became possible. In 1950 a uniform regimen of insecticide spraying was recommended. BHC in a concentration of 11 mg. of gamma BHC per 1 square foot was sprayed in all human habitations and adjacent cattle sheds every 5 weeks during the transmission season, particular importance being given to an early first round to be completed by the end of March when breeding of the vector, Anopheles minimus, was to be expected. BHC was recommended in preference to D D T because its lethal effects were less adversely affected by adsorption on mud walls and because the short intervals between rounds would offset the covering of the insecticide deposit when wails were re-plastered with mud and cowdung, as is customary throughout rural India. The success of the estates' programme is indicated by the Annual Parasite Incidence. In 1954, when the programme of BHC spraying was widely adopted on estates the incidence was 460 per 10,000. In 1963, the last year before estates handed over control to the National Malaria Eradication Programme, it was 1 per 10,000. The National Malaria Eradication Programme (NMEP) was introduced in Assam in 1958, D D T being used in a concentration of 100 rag. per 1 square foot applied twice a year. Tea estates were not included in the spraying programme but could elect to be supplied with D D T , intended to replace BHC which could be held in reserve lest the vector develop resistance to D D T . During 1964 the N M E P took over full responsibility for the attack phase on tea estates. Malaria control that had been successfully carried out for many years by estates with their own staff, equipment, and insecticide was supplanted by malaria eradication which, in 1968, was in its fifth year on tea estates. Some districts were placed in the consolidation phase in 1965 and 1966 but in 1968 reverted, at least in part, to the attack phase. Qnestionaries revealed no consistent pattern and in 1968 an estate might be sprayed once or even twice while neighbouring estates were not sprayed at all. Malaria t r a n s m i s s i o n in A s s a m
The principal vector is A. minimus and transmission has been reported throughout the year except for the winter months of December to March. Breeding is prolific after the early light rains of Spring but decreases when heavy monsoon rains flood the streams and channels. Cases of malaria reach a peak in September or October and thereafter rapidly decline. In three "indicator" estates that are included in this study, in a population of 10,000 there were 16 cases of malaria between January and March 1968, 524 cases in August and September and a total of 836 in the year. A. leucosphyrus (probably A. balabacensis) is a proved secondary vector on a few estates that are close to heavily forested foothills. The importance of A. philippinensis as a vector is still uncertain.
A. B. GILROY
557
Investigations and results To measure the increase of malaria, and its incidence in 1968 on estates that had been free from malaria for several years, a questonary was sent to members of the Assam Branch of the Indian Tea Association asking for this information: The total resident population, workers and dependants The number of fever cases in 1968 The number of blood smears made and examined in 1968 The number of smears reported as positive by the NMEP in each year, 1964-1968 The starting and finishing dates for D D T spraying in 1968 Details of focal spraying of estates in the consolidation phase Details and results of mass blood surveys. Completed questionaries were received from 239 of the 269 tea estates. In Zone 1, the replies covered 94% of the area of tea, in Zone 2, 100% and in Zone 3, 77%. For the total area the coverage was 91%. Briefly, 31 estates reported at least one case of malaria in 1968 but none in 1967 while a further 52 reported an increase of cases in 1968. As no estate has a population of 10,000, and most have" fewer than 5,000, even one case implies an Annual Parasite Incidence (API) exceeding 1 per 10,000. The data are first tabulated for the three Zones of the Assam Branch (Table I). This gives a picture unobscured by detail but it is not evident that only a few estates in each Zone were seriously affected by malaria in 1968. To show this the data are presented for the most malarious estates grouped according to their localities (Table II). TABLE I.
Zone
Population
1
335,734
204,836
174,837
Total
715,407
Malaria cases and annual parasite incidence by Zones Cases & API*
1968
1967
1966
1965
1964
Cases
738
731
567
268
181
API
22.0
21 '8
16 '9
8"0
5-4
Cases
1430
865
711
573
518
API
69"3
42"2
34"7
28"0
25-3
Cases
85
113
36
34
174
API
4"9
6.5
2.1
1"9
10"0
Cases
2253
1709
1314
875
873
API
31-5
23"9
18 "4
12"2
I
12.2
*Annual parasite incidence, cases per 10,000
Malaria cases and the annual parasite incidence in the Zones Table I shows the numbers of cases and the API in each of the three Zones from 1964 to 1968. Combining the Zones, 873 cases in 1964 had increased to 2,253 in 1968; of the total cases in 1968, 33% were in Zone 1, 63% in Zone 2 and only 4% in Zone 3. The API for the combined Zones had increased from 12.2 per 10,000 in 1964 to 31.5 in 1968. The increase was most marked in Zone 2. Why Zone 3 should be relatively D
558
INCREASING MALARIA DURING AN ERADICATION PROGRAMME
malaria-free is not evident from the questionaries. As recently as 1957 the malaria morbidity rate in one district was 18 per 1,000 (GILROY and SCOTT, 1958). The same district reported a morbidity of 0.4 per 1,000 in 1968. The yearly increases of the API are emphasized if the 1964 incidence is taken as unity and the 1968 incidence is expressed as a proportion. In Zone 1, the API in 1968 was 4.1 times greater than in 1964, in Zone 2 it was 2.8 times greater and only in Zone 3 was there a fall from 1.0 in 1964 to 0.5 in 1968. The Tenth Report of the W H O Expert Committee on Malaria (WHO, 1964) reviewing criteria for the start of consolidation recommends that the permissible API should not exceed 0.1 per 1,000 (1 per 10,000). In the Indian programme an API of 2 per 10,000 is accepted as an indication that if it is exceeded in areas in the consolidation phase, they should revert to the attack phase provided that the loci of indigenous transmission are scattered and cannot be cleared by routine remedial measures (DmR, 1968). In 1968 more than 100 tea estates recorded an API higher than 2 per 10,000; on many it was twenty and thirty times higher. Table I shows that in Zone 1, 181 cases in 1964 had increased to 738 in 1968. The increase in Zone 2 was more striking, 518 cases in 1964 and 1,430 in 1968. A total of 2,253 cases for the three Zones in a population of 790,000 would have been unbelievably few 40 years ago but not in the fifth year of the National Eradication Programme. M a l a r i a on s e l e c t e d e s t a t e s o f h i g h e n d e m i c i t y
Table I I gives the number of cases and the annual parasite incidences for 38 estates selected because of their high API and grouped in 6 localities. In 1968 the lowest API was 47 per 10,000 and the highest 301. Of the total resident population of the three combined Zones, 16% live on these 38 estates which produced 87% of the malaria in 1968. For all 38 estates the API increased slowly from 46 per 10,000 in 1964 to 99 in 1967 and thence sharply to 174 in 1968. I f the API of 1964 is taken as unity, the annual increase was 1.2 times in 1965, 1.5 times in 1966, 2.2 times in 1967 and 3.8 times in 1968. Although malaria in 1968 was a serious problem on only 38 of the 239 estates that completed the questionary, two ominous facts appear, the API is increasing more sharply and these 6 localities are widely separated along 250 miles of the south bank of the Brahmaputra river. The questionaries do not give information to explain the considerable differences in the incidence of malaria in the groups, ranging in 1968 from 47 to 301 per 10,000. For that, epidemiological and entomological investigations would be necesasry. S u r v e i l l a n c e o n tea estates
Tea estates must by law provide hospitals and medical staff for workers and their families. Full use is made of medical services, people coming for treatment of the most minor ailment. This facilitates efficient passive case detection and treatment, both presumptive and radical. Microscope slides and anti-malarial drugs are supplied by the N M E P and blood smears made on the estates are collected by surveillance workers and examined in the N M E P laboratories. 61,512 blood smears from tea estates were examined in 1968 and 2,253 were positive for malaria, a slide positivity rate of 3.7%. The completeness of blood smear examination is indicated by the Annual Blood Examination Rate (ABER) which expresses the
A. B. GILROY
559
number of blood smears in the year as a percentage of the relevant population. DmR (1968) accepts an ABER between 5% and 10% of the population in hyperendemic areas as reasonable if remedial measures are prompt and adequate. TABLE II.
Malaria cases and annual parasite incidences for groups of selected estates
Locality of of group Population Doom Dooma
18,671
Sonari
20,526
Jorhat
30,223
Golaghat
11,859
Nowgong
24,879
Tezpur
Total
6,754
112,912
*API--Annual parasite incidence: cases per 10,000. Note. For convenience of description the localities of the groups are given the name of the nearest township. Table I I I gives data for passive surveiUance both for the six groups of 38 malarious estates and for the three Zones. The ABER was 6.9% in Zone 1, 10.8% in Zone 2 and 9.3% in Zone 3. The ABER will be reduced by the inclusion of estates where malaria is only a minor problem and where, consequently, little interest is taken in passive case detection. On the other hand, in the groups of seriously affected estates the ABER ranges between 13% and 28% compared with 10% or less in the Zones. The slide positivity rates are evidence of the wide extent and volume of the parasite reservoir. As shown in Table I I I in one group where 3,714 smears were examined, 14-9% were positive and of 22,054 smears made in Zone 2, 6.5% were positive.
INCREASINGMALARIADURINGAN ERADICATIONPROGRAMME
560
TABLE III. Passive case detection 1968 for the three Zones and selected estates of Table II
Population
Fever cases
No. of blood smears
Doom Dooma
18,671
3,714
3,714
20
14.9
Sonari
20,526
3,135
2,603
13
6.8
Jorhat
30,223
6,301
6,211
21
2-7
Golaghat
11,859
4,464
3,370
28
10"6
Nowgong
24,879
5,391
4,950
20
13-6
6,754
1,157
1,157
17
2.8
Zone 1
336,000
31,041
23,119
6"9
3.2
Zone 2
204,900
28,304
22,054
10.8
6"5
Zone 3
174,900
24,853
16,339
9-3
0-5
Locality of group
Tezpur
ABER* per cent
Slide positivity rate per cent
Zone
Note.
The groups are included in their Zones. *Annual blood examination rate---number of blood smears expressed as a percentage of the relevant population.
DDT spraying cycles in 1968 The first round of spraying was scheduled to start on 15 May and to be completed by the end of June; the second round was to be applied between the middle of August and the end of September. In the event the starting dates were seriously delayed and D D T was not applied until the transmission season was established and secondary cases were occurring. On many estates the first round was started after the second should have been finished and of 149 estates that were sprayed in 1968, 34 were sprayed for the first time in November when malaria transmission had ceased. Even had the proposed time-table been observed, a first round of D D T starting in the middle of May is too late to prevent transmission in April and the secondary cases that are the true origin of epidemics during eradication (MACDONALD 1957).
Summary Data obtained by questionary have established the increase, year by year, of malaria on tea estates. It has been measured by the almual parasite incidence and the extent of the parasite reservoir has been estimated by the annual slide positivity rate. The most seriously affected estates are confined to six localities, far apart and separated by villages and by tea estates where there has not been an increase of malaria. D D T spraying in 1968 was delayed until long after the transmission of malaria was established. Passive case detection was adequate and approached, and even exceeded, the standard of 10% of the population as measured by the annual blood examinauon rate. The increase of malaria over an ever wider area is affecting people whose immunity has been lost during years of freedom from attacks.
A. B. GILROY
561
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